Icy Travelers Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.

When used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows). Young surfaces exhibit few impact craters and are typically varied and complex. In contrast an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old.

When used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows). Young surfaces exhibit few impact craters and are typically varied and complex. In contrast an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old.